This invention relates to inertial or dead reckoning navigation, in particular to that of aircraft.
There is a need for a low-cost navigation system to act effectively and inexpensively as a backup in the vent of failure to receive and process reliable Global Positioning System (GPS) navigation signals. This is particularly important for single-engine General Aviation (GIA) applications, where cost is often the overriding consideration. The United States Government is planning to phase out existing ground-based navigation systemsxe2x80x94the VOR (VHF Omni-Directional Range) and typically associated DME (Distance Measuring Equipment) network, the ILS (Instrument Landing System), and non-directional beacons (NDE). Replacing these legacy systems with GPS will realize considerable cost savings. However, this transition leads to the need for a reliable, autonomous, and inexpensive backup system should the GPS signals or airborne GPS receiving equipment be lost or compromised. For national security reasons, the Government may at any time re-impose Selective Availability (S/A), whereby the GPS accuracy is deliberately degraded. Continuation of safe flights under Instrument Meteorological Conditions (IMC) with only GPS capability would be impossible. Scheduled airline aircraft are typically equipped with inertial navigation systems (INS), which would be available as a backup. Modern INS generally use expensive optical fiber gyros. However, existing INS are too costly for typical G/A installations, particularly those involving light, piston-powered aircraft. Furthermore, a backup system must operate without any external signals or sensors. There is no guarantee that any particular ground-based dedicated or chance radiator or group of radiators will be available in an emergency. Under IMC, there is also no guarantee that usable surface features will be visible and identifiable, even with advanced terrain and feature recognition devices.
Conventional INS employ a combination of accelerometers and rotation sensors. Modern strapdown INS use laser optical gyros to sense rotation.
U.S. Pat. No. 5,272,639 to McGuffin teaches a terrain reference navigation system was that extended the then prior art of terrain aided INS by incorporating gravity and geomagnetic field data as well. The then prior art used radar and barometric altimeters to compare measured ground elevation to that stored essentially as a topographical map. To this the method described in U.S. Pat. No. 5,272,639 added maps of the gravity and geomagnetic fields. Thus measurements of the local gravity and geomagnetic fields were to be correlated to map data to update and correct the INS position. The INS was otherwise conventional. In my present invention the conventional INS and elevation measuring equipment are eliminated completely.
U.S. Pat. No. 5,319,561 to Matsuzaki teaches a system for determining the instantaneous heading of a vehicle using geomagnetic and turning rate sensors. The latter sensors may be gyroscopic (including mechanical and optical gyroscopes) or wheel sensors. The primary object of this invention is to correct for magnetic compass errors in vehicle heading determination.
U.S. Pat. No. 6,208,936 to Minor and Rowe teaches the use of a geomagnetic field sensor and knowledge of the local geomagnetic field to determine the rotation of a spinning vehicle (such as a rocket) to computationally de-spin the navigation solution for a MEMS-IMU/GPS (Micro-Electro-Mechanicalxe2x80x94Inertial Measurement Unit) navigation system and thus maintain a stable solution.
In accordance with the present invention a geophysical INS (Inertial Navigation System) comprises a set of sensors that are strapped down to a vehicle such as an aircraft and are responsive to the geomagnetic field and to gravity and vehicle acceleration. Changes in the sensed magnetic field components and vehicle acceleration components are the input state variables for a predictive filter, such as a Kalman filter, whose outputs are used to estimate the velocity and geographical location of the vehicle. In effect, magnetic field sensors replace the gyroscopes in a conventional INS. Stored magnetic and gravity field data (e.g., maps) then provide new values of the local geophysical parameters. The system is intended primarily to be a low-cost stand-alone backup for navigation systems such as GPS (Global Positioning System).
Objects and Advantages
Accordingly, the overall object and advantage of this invention is to provide a lower cost means for inertially navigating a vehicle such as an aircraft than provided by an INS using mechanical and optical gyroscopic rotation sensors.
Another object and advantage of this invention is to provide a stand-alone backup means for navigating aircraft and other vehicles in the event of failure or degradation of the GPS.
A further object and advantage of this invention is to provide inertial navigation capability to light aircraft.
Still further objects and advantages will become apparent from a consideration of the ensuing description and accompanying drawings.